Abstrict A hot-wire type air flow meter for an internal combustion engine
has an exothermic resistor film provided on a thin portion formed
on a silicon substrate and a control circuit for controlling a voltage
to be applied to the resistor film or a current to be supplied.
The resistor film is arranged in a suction pipe of the internal
combustion engine and heat is emitted to an air through the resistor
film. In the case where a liquid droplet is deposited onto the resistor
film, the control circuit controls the applied voltage to a value
smaller than a voltage which is applied to the resistor film at
the time of measuring a maximum specified measuring range or controls
the supply current to a value smaller than a current flowing in
the resistor film at the time of measuring the maximum specified
measuring range.
Claims What is claimed is:
1. A hot-wire type air flow meter for an internal combustion engine,
comprising: an exothermic resistor film provided on a thin portion
formed on a silicon substrate arranged in a suction pipe of the
internal combustion engine and emits heat to an air through said
resistor film; and a control circuit for controlling a voltage to
be applied to said resistor film or a current to be supplied, said
control circuit being configured such that, where a liquid droplet
is deposited onto said resistor film, said applied voltage is controlled
to a value smaller than a voltage which is applied to said resistor
film at the time of measuring a maximum measuring of an air flow
rate within said suction pipe when a liquid droplet is not deposited
onto said resistor film range or said supply current is controlled
to a value smaller than a current flowing in said resistor film
at the time of measuring the maximum measuring range.
2. A meter according to claim 1 further comprising another exothermic
resistor provided on said silicon substrate in a portion other than
said thin portion.
3. A hot-wire type air flow meter for an internal combustion engine,
comprising: a first resistor film provided on a thin portion formed
on a silicon substrate and arranged in a suction pipe of the internal
combustion engine to emit heat to a measuring fluid through said
first resistor film and a second resistor film whose resistance
value changes in accordance with an ambient temperature; and a control
circuit which has a bridge circuit including said second resistor
film and controls a voltage to be applied to said first resistor
film or a current to be supplied, said control circuit being configured
such that, where an output of said bridge circuit is larger than
a specified value, said applied voltage is controlled to be smaller
than a voltage which is applied to said first resistor film when
measuring a maximum measuring range of an air flow rate within said
suction pine when a liquid droplet is not deposited onto such first
resistor film or said supply current is controlled to a value smaller
than a current flowing in said first resistor film when measuring
the maximum measuring range.
4. A meter according to claim 3 further comprising another exothermic
resistor provided on said silicon substrate in a portion other than
said thin portion.
5. A hot-wire type air flow meter for an internal combustion engine,
comprising: an exothermic resistor arranged in a measuring fluid
and emitting heat to a measuring fluid; a control circuit for controlling
a voltage to be applied to said resistor or a current to be supplied;
first limiting means for limiting said applied voltage or said supplied
current to a value which is equal to or less than a first value;
and second limiting means for limiting said applied voltage or said
supplied current to a value which is equal to or less than a second
value smaller than said first value, said control circuit being
configured such that, in the case where a liquid droplet is deposited
onto said resistor, said applied voltage or said supplied current
is limited to a value which is equal to or less than said second
value.
6. A meter according to claim 5 further comprising changing means
for changing a predetermined value or said second value on the basis
of a temperature of the liquid droplet or a measuring fluid.
7. A meter according to claim 5 further comprising changing means
for changing said predetermined value or said second value on the
basis of an output of a resistor which is arranged in said measuring
fluid and whose resistance value changes in accordance with an ambient
temperature.
8. A meter according to claim 7 wherein said changing means changes
the value so as to reduce said predetermined value or said second
value when said resistance value increases or changes the value
so as to increase said predetermined value or said second value
when said resistance value decreases.
9. A meter according to claim 5 wherein said exothermic resistor
is a thin film resistor provided on a thin portion formed on a silicon
substrate and arranged in a suction pipe of the internal combustion
engine.
10. A meter according to claim 9 further comprising another exothermic
resistor provided on said silicon substrate in a portion other than
said thin portion.
11. A meter according to claim 10 wherein a heat generation of
said another exothermic resistor is controlled independent of said
exothermic resistor.
12. A hot-wire type air flow meter for an internal combustion engine,
comprising: an exothermic resistor arranged in a measuring fluid
and emitting heat to said measuring fluid; and a control circuit
for controlling a voltage to be applied to said resistor or a current
to be supplied, said control circuit being configured such that,
in the case where a liquid droplet is deposited onto said resistor,
said control circuit controls said applied voltage or said supplied
current so that a generation heat amount per unit area of said resistor
is smaller than an amount of heat by which a burn-out occurs at
an interface between said resistor and said liquid droplet.
13. A hot-wire type air flow meter for an internal combustion engine,
comprising: an exothermic resistor arranged in a measuring fluid
and emitting heat to said measuring fluid; and a control circuit
for controlling a voltage to be applied to said resistor or a current
to be supplied, said control circuit being configured such that,
in the case where a liquid droplet is deposited onto said resistors,
said control circuit controls said applied voltage or said supplied
current so that a generation heat amount per unit area of said resistor
is smaller than a predetermined value.
14. A meter according to claim 13 wherein said predetermined value
is equal to 4.times.10.sup.5 W/m.sup.2.
15. A meter according to claim 13 further comprising changing
means for changing said predetermined value or said second value
on the basis of a temperature of the liquid droplet or said measuring
fluid.
16. A meter according to claim 13 further comprising changing
means for changing said predetermined value or said second value
on the basis of an output of a resistor which is arranged in said
measuring fluid and whose resistance value changes in accordance
with an ambient temperature.
17. A meter according to claim 16 wherein said changing means
changes the value so as to reduce said predetermined value or said
second value when said resistance value increases or changes the
value so as to increase said predetermined value or said second
value when said resistance value decreases.
18. A hot-wire type air flow meter for an internal combustion engine,
comprising: a first resistor which is arranged in a measuring fluid
and generates heat emitted to said measuring fluid, and a second
resistor whose resistance value changes in accordance with an ambient
temperature; a control circuit which has a bridge circuit including
said second resistor and controls a voltage to be applied to said
first resistor or a current to be supplied; first limiting means
for limiting said applied voltage or said supply current to a value
which is equal to or less than a first value; and second limiting
means for limiting said applied voltage or said supply current to
a value which is equal to or less than a second value smaller than
said first value, said control circuit being configured such that,
in the case where an output of said bridge circuit is larger than
a predetermined value, said control circuit limits said applied
voltage or said supplied current to be equal to or less than said
second value.
19. A meter according to claim 18 further comprising changing
means for changing a predetermined value or said second value on
the basis of a temperature of a liquid droplet or said measuring
fluid.
20. A meter according to claim 18 further comprising changing
means for changing a predetermined value or said second value on
the basis of an output of a resistor which is arranged in said measuring
fluid and whose resistance value changes in accordance with the
ambient temperature.
21. A meter according to claim 20 wherein said changing means
changes the value so as to reduce said predetermined value or said
second value when said resistance value increases or changes the
value so as to increase said predetermined value or said second
value when said resistance value decreases.
22. A hot-wire type air flow meter for an internal combustion engine,
comprising: a first resistor which is arranged in a measuring fluid
and generates heat emitted to said measuring fluid, and a second
resistor whose resistance value changes in accordance with an ambient
temperature, and a control circuit which has a bridge circuit including
said second resistor and controls a voltage to be applied to said
first resistor or a current to be supplied, said control circuit
being configured such that, in the case where an output of said
bridge circuit is larger than a certain value, said control circuit
controls said applied voltage or said supplied current to a value
which is smaller than a heat amount by which a burn-out occurs at
an interface between said first resistor and a liquid droplet.
23. A hot-wire type air flow meter for an internal combustion engine,
comprising: a first resistor which is arranged in a measuring fluid
and generates heat emitted to said measuring fluid, and a second
resistor whose resistance value changes in accordance with an ambient
temperature; and a control circuit which has a bridge circuit including
said second resistor and controls a voltage to be applied to an
exothermic resistor or a current to be supplied, said control circuit
being configured such that, in the case where an output of said
bridge circuit is larger than a certain value, said control circuit
controls said applied voltage or said supplied current so that a
generation head amount per unit area of said first resistor is smaller
than a predetermined value.
24. A meter according to claim 23 wherein said predetermined value
is equal to 4.times.10.sup.5 W/m.sup.2.
25. A meter according to claim 23 further comprising changing
means for changing said predetermined value or said second value
on the basis of a temperature of a liquid droplet or said measuring
fluid.
26. A meter according to claim 23 further comprising changing
means for changing said predetermined value or said second value
on the basis of an output of a resistor which is arranged in said
measuring fluid and whose resistance value changes in accordance
with the ambient temperature.
27. A meter according to claim 26 wherein said changing means
changes the value so as to reduce said predetermined value or said
second value when said resistance value increases or changes the
value so as to increase said predetermined value or said second
value when said resistance value decreases.
28. A meter according to claim 25 wherein said changing means
changes the value so as to reduce said predetermined value or said
second value when said temperature increases and changes the value
so as to increase said predetermined value or said second value
when said temperature decreases.
29. A hot-wire type air flow meter for an internal combustion engine,
comprising an exothermic resistor arranged in a suction pipe of
said internal combustion engine wherein heat is emitted to an air
through said resistor; a control circuit for controlling a voltage
to be applied to said resistor or a current to be supplied; and
transmitting means for transmitting a specific signal to a control
unit of said internal combustion engine in the case where a liquid
droplet is deposited onto said resistor.
30. A meter according to claim 29 wherein said transmitting means
transmits a signal including information regarding an amount of
said deposited liquid droplet to the control unit of said internal
combustion engine.
31. A meter according to claim 29 wherein said exothermic resistor
is a thin film resistor provided on a thin portion formed on a silicon
substrate and arranged in said suction pipe of said internal combustion
engine.
32. A meter according to claim 29 further comprising another exothermic
resistor provided on a silicon substrate in a portion other than
said thin portion.
33. A meter according to claim 32 wherein a heat generation of
said another exothermic resistor is controlled independent of said
exothermic resistor.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a measuring apparatus of an air flow rate
and, more particularly, to a hot-wire type air flow meter (heat
flow meter) suitable for detection of a suction air flow rate of
an internal combustion engine.
2. Description of Related Art
Hitherto, as an air flow rate measuring apparatus which is provided
for an electronic control type fuel injection apparatus of an internal
combustion engine such as an automobile or the like and measures
a suction air flow rate, a number of hot-wire type apparatuses have
been used because a mass air flow rate can be directly detected.
In this instance, to assure a reliability of an exothermic resistor,
a sensor having a certain extent of heat capacity which is constructed
by winding a platinum wire around a bobbin and coating it with glass,
forming a thin film resistor onto a ceramic substrate, or the like
is often used. In a sensor having a large heat capacity, although
a system such that a current is limited when a waterdrop is deposited
onto a portion other than an internal combustion engine has been
shown in JP-A-5-264312 a power voltage is large to be 24V and a
measuring region of a rated value or more is limited. Such a system
is used, particularly, for measurement of a flow rate of the compressed
air and there is a problem that although a waterdrop is instantaneously
evaporated usually at 100.degree. C. at an atmospheric pressure,
in the compressed air, an evaporation temperature of the water is
high, it takes time to evaporate the waterdrop, and an excessive
current flows in an exothermic portion. In this instance, since
the power voltage is high to be 24V, when a waterdrop is deposited,
a current which is one or more digits larger than a rated current
flows, and accordingly, it is limited by a current limiter. In an
internal combustion engine, to prevent a fusion of a hot wire, a
system for reducing an applied voltage to a bridge by detecting
a voltage across a hot wire resistor has been disclosed in JP-A-61-102522
or the like. According to such a system, to prevent the fusion due
to the supply of the abnormal voltage because of noises or the like,
a voltage limit value larger than the maximum value at the time
of the stationary operation is set.
As a flow meter having a high response speed, a number of flow
meters in which an exothermic resistor of a small heat capacity
is attached on a silicon substrate have been proposed. For example,
as shown in JP-B-5-7659 a thin film is formed on a silicon semiconductor
substrate and a flow rate is detected by a thermal sensor.
SUMMARY OF THE INVENTION
In the conventional techniques mentioned above, in case of using
an exothermic resistor of a large heat capacity, a response speed
at the time of a power-on of an air flow rate measuring apparatus
is determined only from the relation between a power source voltage
and the heat capacity of the exothermic resistor. In an internal
combustion engine such as an ordinary gasoline engine or the like,
therefore, since a power source voltage is usually small to be about
12V, an operating current to the exothermic resistor is limited
and it takes time to heat, so that it is difficult to accurately
measure an air flow rate for a predetermined time at the start of
an engine of an automobile or the like.
To improve the response speed, it is necessary to use an exothermic
resistor of a small heat capacity. In a hot-wire air flow meter,
however, the maximum flow rate which can be measured is limited
unconditionally to a certain degree by the heat capacity and a heating
temperature of the exothermic resistor which is used for measurement
and by a power source voltage which can be applied. It is because
when the heat capacity is small, if it is intended to raise a measuring
sensitivity (maximum flow rate) of the flow rate by increasing the
heating temperature, a limitation such that the exothermic resistor
deteriorates when the heating temperature is equal to or higher
than a predetermined value occurs. For example, a system such that
an exothermic resistor is constructed by forming a thin film with
a diaphragm structure onto a silicon substrate and a temperature
of the exothermic resistor is controlled so as to become a constant
temperature by a feedback circuit by using a bridge circuit or the
like has the highest response speed. If a waterdrop, an oil droplet,
or the like is deposited onto the exothermic resistor portion, however,
a heating balance is broken and a voltage is applied so that the
heating temperature of the exothermic resistor locally rises. Even
in case of the applied voltage which does not cause a problem in
the ordinary operation, therefore, if the temperature elevation
continues, there is a possibility that the resistor of a small heat
capacity constructed by the thin film is deteriorated by the heat
and, in the worst case, the diaphragm is broken by a thermal stress.
Although the excessive temperature elevation can be prevented by
limiting the voltage applied to the hot-wire type resistor so as
not to locally raise the heating temperature of the exothermic resistor,
there is a problem that a measuring range of the inherent flow rate
is also narrowed.
It is, therefore, an object of the invention to provide a hot-wire
type air flow meter for an internal combustion engine which can
solve the problems of the conventional techniques mentioned above.
According to an aspect of the invention, there are provided: means
for detecting a heating state of an exothermic resistor by distinguishing
a case where a waterdrop, an oil droplet, or the like is deposited
onto the exothermic resistor portion from a normal case where there
is no deposition; and circuit means for limiting a heating time
by the detected heating state. Thus, only in the case where the
waterdrop, oil droplet, or the like is deposited onto the exothermic
resistor portion, the heating state is limited without narrowing
a measuring range of an inherent flow rate and a deterioration of
the resistor can be prevented.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a circuit diagram showing an example of a hot-wire type
flow meter according to the first embodiment of the invention;
FIGS. 2A and 2B are diagrams each showing the limiting operation
at the time when a waterdrop is deposited and a flow rate measuring
range;
FIG. 3 is a diagram showing output voltage characteristics of a
bridge potential difference and a differential amplifier;
FIGS. 4A to 4D are comparison diagrams of the voltage limiting
operations of a bridge voltage at the time of power-on;
FIG. 5 is a diagram showing a burn-out region according to a temperature
and a hot flow rate;
FIGS. 6A and 6B are comparison diagrams of the voltage limiting
operations of the bridge voltage at the time of power-on in the
case where it has a temperature characteristic;
FIG. 7 is a diagram of an example showing a pattern of a resistor
formed on a silicon substrate;
FIG. 8 is a cross sectional view of an example of the resistor
formed on the silicon substrate;
FIG. 9 is a circuit diagram showing an example of a hot-wire type
air flow meter according to the second embodiment of the invention;
FIGS. 10A to 10F are comparison diagrams of the voltage limiting
operations of a bridge voltage at the time of power-on;
FIG. 11 is a diagram showing an example of a pattern of a resistor
formed on a silicon substrate;
FIG. 12 is a circuit diagram showing an example of a hot-wire type
flow meter according to the third embodiment of the invention;
FIG. 13 is a diagram showing a state where a resistor formed on
a silicon substrate is mounted on a circuit board; and
FIG. 14 is a circuit diagram showing an example of a hot-wire type
flow meter according to the fourth embodiment of the invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
An embodiment of a hot-wire type air flow meter for an internal
combustion engine according to the invention will be described in
detail hereinbelow. In the following drawings, component elements
having the same or similar functions are designated by the same
reference numerals.
An example of a construction of a hot-wire type air flow meter
according to the first embodiment of the invention will be described
hereinbelow with reference to FIG. 1. A hot wire driving circuit
I is connected to a power source 101 and generates a signal corresponding
to an air flow rate. A hoisten-bridge circuit is constructed by:
an exothermic resistor 11; a temperature compensation resistor (a
resistor for measuring a suction air temperature, a temperature
detecting element of a fluid, a temperature sensor of an air and
a medium, a fluid temperature sensing unit, a temperature dependent
resistor) 12 as a resistor whose resistance value varies in accordance
with an ambient air temperature; and resistors 13 and 14. The hot
wire driving circuit 1 is constructed in such a manner that a current
flowing in the exothermic resistor 11 is adjusted by a differential
amplifier 15 and a transistor 16 so that a potential difference
at a middle point of a bridge is equal to zero by the hoisten-bridge
circuit. When a heating temperature of the exothermic resistor 11
is low, an output of the differential amplifier 15 is large and
the resistor 11 operates so as to heat more. With this construction,
the resistance value of the exothermic resistor 11 is controlled
so as to be constant irrespective of an air flow rate, namely, in
a manner such that the temperature is set to a predetermined value.
In this instance, a signal corresponding to an air flow rate due
to the exothermic resistor 11 is inputted to a zero span circuit
2. The zero span circuit 2 is constructed by a differential amplifier
21 and resistors 22 23 24 25 26 and 27.
The exothermic resistor 11 can be constructed by, for example,
a thin or thick film of platinum or tungsten as an exothermic body
onto a plate-shaped glass or ceramic substrate. It is, however,
necessary to design it so as to have a small heat capacity.
Particularly, to reduce the heat capacity and raise a response
speed, it is desirable to construct the exothermic resistor 11 in
such a manner that a thin or thick film of platinum or tungsten,
a polysilicon resistor, or a resistor of monocrystal silicon is
formed as an exothermic body onto a thin portion of a semiconductor
substrate of silicon or the like.
The exothermic resistor 11 is provided in a suction passage of
an internal combustion engine of an automobile or the like and a
voltage output corresponding to a flow rate of the air flowing in
the suction passage is obtained as an output of the differential
amplifier 21. An ordinary hot-wire type air flow meter is constructed
as mentioned above.
A measuring range of the air flow rate in case of preventing a
deterioration, breakage, or the like of the resistor due to overheating
will now be described with reference to FIG. 2A. This diagram shows
the relation between the flow rate and a voltage (or current) which
is applied to the exothermic resistor due to an increase in flow
rate. An output according to the flow rate becomes an output of
the hot-wire type air flow meter. Now, considering an operating
state at the time when a waterdrop is deposited, it is necessary
to limit the voltage to a value which is equal to or less than a
voltage (or current) at which a deterioration, breakage, or the
like is caused in the resistor by the deposition of the waterdrop.
A limiter limits the voltage to a value which is equal to or less
than such a voltage. In this case, by limiting the voltage by the
limiter, a specified measuring range of the air flow rate is simultaneously,
limited. Particularly, in case of using an exothermic resistor of
a small heat capacity constructed by forming a thin film onto a
silicon semiconductor substrate, since the voltage (or current)
at which a deterioration, breakage, or the like is caused in the
resistor by the deposition of the waterdrop drops, there is a tendency
such that the measuring range is narrowed.
On the other hand, to widen the measuring range to the voltage
(or current) at which a deterioration, breakage, or the like is
caused in the resistor by the deposition of the waterdrop, it is
possible to cope with such a purpose by detecting a depositing state
of the waterdrop and limiting the voltage (or current) in accordance
with the presence or absence of the waterdrop. FIG. 2B shows a measuring
range of the air flow rate in this instance. Particularly, in case
of using an exothermic resistor of a small heat capacity constructed
by forming a thin film onto a silicon semiconductor substrate, to
widen the measuring range to the voltage (or current) at which a
deterioration, breakage, or the like is caused 64 in the resistor
by the deposition of the waterdrop, it is necessary to increase
a rated value and set a limiting condition (first limiter) such
as a voltage (or current) or the like to a rated value (maximum
specified measuring range) or more. In this state, however, since
a breakage or the like is caused at the time of deposition of a
waterdrop, second limiting means for limiting the voltage to a value
which is equal to or less than a rated value at which a breakage
or the like is not caused by the voltage (or current) when a deposition
of a waterdrop is detected is provided. Particularly, as a level
of the second limiter, it is especially effective to set the voltage
to a level so as not to cause a state called "burn-out"
such that when a waterdrop is deposited, an air layer is formed
on the surface of the exothermic resistor and overheating occurs.
As shown in FIG. 1 in the embodiment, a Zener diode 44 is provided
as a first limiter for limiting the voltage within the measuring
range of the flow rate or more. The Zener diode limits the voltage
when a battery voltage of the power source 101 rises, and it does
not operate ordinarily.
In this instance, particularly, a differential amplifier 68 is
prepared as second bridge detecting means for detecting a potential
difference at a middle point of the bridge in order to detect the
depositing state of the waterdrop, and a voltage limitation switching
circuit 6 is provided as a second limiter for limiting an output
voltage V3 of the differential amplifier 15 in accordance with a
detection result. FIG. 3 shows characteristics of the bridge potential
difference and the output voltage of the differential amplifier
as input/output characteristics of the bridge circuit.
Usually, the current flowing in the exothermic resistor 11 is adjusted
by the differential amplifier 15 and transistor 16 so that an output
(potential difference at the middle point of the bridge) of the
bridge circuit is equal to zero. It is because the voltage which
is applied to the bridge circuit is controlled within a range from
0 to the power source voltage by a feedback in accordance with the
potential difference at the middle point of the bridge. When the
potential difference input at the middle point of the bridge is
equal to or larger (or smaller) than a predetermined value, the
output of the differential amplifier 15 is limited to the power
source voltage or grounding level.
In the ordinary operation, since there is an output sensitivity
only for the potential difference (+dV) of the bridge on the plus
side, it is difficult to distinguish the bridge potential difference
(+dV) when the waterdrop is deposited from the bridge potential
difference (+dV) which is temporarily generated at the time of the
maximum flow rate. Therefore, attention is paid to the fact that
a large bridge voltage is continuously generated when the waterdrop
is deposited, and the output voltage is limited by the limiter only
in this case.
Since it is actually difficult to limit the output voltage only
by the differential amplifier 15 the differential amplifier 68
is added and used as a detector of the waterdrop and as a limiter.
Specifically speaking, the differential amplifier 68 is constructed
in such a manner that a polarity of the potential difference at
the middle point of the bridge is changed and (-dV) is inputted,
namely, the potential difference at the middle point of the bridge
is changed to the opposite polarity and (-dV) is inputted, when
the large bridge potential difference (+dV) is continuously generated,
the output of the differential amplifier 68 is set to the grounding
level and the second limiter is made operative, and the second limiter
is made inoperative in the other cases. Since there is generally
an offset voltage in the input of the differential amplifier, a
partial region of the bridge potential difference crosses.
As mentioned above, for the ordinary control voltage range (only
from 0V to the power source voltage and the plus side) of the voltage
to be applied to the bridge circuit, the potential difference at
the middle point of the bridge is outputted as a differential amount
between the plus side and the minus side. Therefore, the polarities
on the plus side and the minus side of the potential difference
at the middle point of the bridge are shared by the two differential
amplifiers 15 and 68 and such a potential difference is separately
inputted and fed back to the voltage which is applied to the bridge
circuit. Consequently, if a state where the potential difference
at the middle point of four resistors constructing the bridge circuit
is equal to, for example, 100 mV or more or 100 mV or less continues,
it is possible to determine that such a state is a certain specific
state (for example, waterdrop depositing state).
The voltage limitation switching circuit 6 is constructed by: resistors
61 62 and 63 connected serially between a battery voltage VB of
the power source 101 and the ground; a Zener diode 67 connected
to a node of the resistors 62 and 63; diodes 65 and 66 connected
to a node of the resistors 61 and 62; and the like. The voltage
limitation switching circuit 6 limits the output of the differential
amplifier 15 to a voltage V7 which is set by the Zener diode 67
through a resistor 17 the diode 66 and the resistor 62. If a resistor
having a temperature coefficient which is equal to or larger than
that of the temperature compensation resistor 12 is used as a resistor
62 and arranged in a suction pipe, the temperature characteristic
can be also provided at the voltage V7 which is set by the Zener
diode 67.
An example will now be specifically described. In the case where
the exothermic resistor 11 is controlled,at a constant temperature
in the ordinary operation, a potential difference dV at the middle
point of the bridge approaches zero. Assuming that the offset voltage
of the differential amplifier 15 is equal to plus several mV, dV
is stabilized at a level of minus several mV. Even in the case where
the flow rate increases and potential difference dV slightly increases,
by rapidly increasing the heating current of the exothermic resistor
11 on the basis of the output of the differential amplifier 15 and
always making the constant temperature control, the potential difference
dV at the middle point of the bridge is held at the level of minus
several mV near zero. In this instance, assuming that the potential
difference dV at the middle point of the bridge is held at the level
of minus several mV, the output of the differential amplifier 68
to detect the depositing state of the waterdrop is set to the high
level. Assuming that both of the differential amplifier 15 and differential
amplifier 68 are connected to the power source 101 by similar circuit
constructions, an output V3 of the differential amplifier 15 is
not subjected to the voltage limitation by the operations of the
diodes 65 and 66 of the voltage limitation switching circuit 6.
On the other hand, when a waterdrop is deposited, since the heating
temperature of the exothermic resistor 11 does not rise, the large
potential difference dV at the middle point of the bridge of plus
hundreds of mV is generated. When this state is detected, the output
of the differential amplifier 68 is set to the low level. The output
V3 of the differential amplifier 15 is subjected to the voltage
limitation by the voltage V7 set by the resistors 61 62 and 63
and Zener diode 67 by the operation of the voltage limitation switching
circuit 6.
Subsequently, the details of the voltage limiting operation using
the voltage limitation switching circuit 6 will be described with
reference to FIGS. 4A to 4D. FIGS. 4A and 4B show the operations
of the potential difference dV at the middle point of the bridge
and the output V3 of the differential amplifier 15 in the normal
operation, respectively. The output V3 of the differential amplifier
15 becomes maximum and the potential difference dV also increases
for a time T1 during which the exothermic resistor 11 is heated
upon activation. After that, when the initial heating is finished,
the stable constant temperature control operation is performed after
the elapse of a time T2 the potential difference dV decreases,
and the output V3 of the differential amplifier 15 is stabilized
to a predetermined voltage according to the flow rate. For example,
in case of the exothermic resistor of the small heat capacity constructed
by forming the thin film onto the silicon semiconductor substrate,
the time T2 which is required until the control operation is stabilized
after the end of the initial heating can be set to a short time
within a range from several msec to tens of msec in accordance with
the heat capacity.
In the voltage limiting operation in FIGS. 4C and 4D, a difference
due to the presence or absence of the voltage limiting operation
when a waterdrop is deposited according to the embodiment will be
described. First, a case of no voltage limitation will be explained.
When a waterdrop is deposited, the output V3 of the differential
amplifier 15 becomes the maximum voltage upon activation and its
state is almost maintained although there is a slight fluctuation
until the water is evaporated and the ordinary operation is performed
(refer to FIG. 4D). For this period of time, although the potential
difference dV at the middle point of the bridge increases upon activation,
the unstable operation such that it is decreased and increased is
repeated after that (refer to FIG. 4C). It is because if the voltage
applied to the bridge is large, when a waterdrop is deposited, an
air layer is partially formed on the surface of the exothermic resistor
and a state called "burn-out" corresponding to overheating
occurs. The exothermic resistor repeats the quenching by the waterdrop
and the partial high temperature heating state by the air layer.
The potential difference dV at the middle point of the bridge also
repetitively increases and decreases in accordance with the heating
state of the exothermic resistor. The reason why the potential difference
dV increases and decreases is because the heating temperature rises
due to the formation of the air layer and such a state is temporarily
recognized as an ordinary balanced control situation of the bridge.
In this state, if the potential difference dV at the middle point
of the bridge is merely checked, the depositing state of the waterdrop
cannot be discriminated. If the temperature continuously rises,
the resistor of the small heat capacity formed by the thin film
is deteriorated by the heat or the breakage of the diaphragm cannot
be prevented due to the heat stress.
On the other hand, a case where there is a voltage limitation will
be described. When the waterdrop is deposited, if the potential
difference dV at the middle point of the bridge is large, the output
V3 of the differential amplifier 15 is limited by the voltage limitation
switching circuit 6. That is, when the waterdrop is deposited, the
bridge applied voltage is limited to a predetermined value or less
so that an electric power can be limited to a predetermined power
corresponding to the area of the exothermic resistor or less so
as not to cause a state called "burn-out" indicative of
an overheating in which the air layer is formed on the surface of
the exothermic resistor. Thus, if the state where the potential
difference dV at the middle point of the bridge is large continues
and there is no waterdrop, the potential difference dV decreases
and the voltage limitation is cancelled. Therefore, by limiting
the bridge applied voltage in accordance with the potential difference
dV upon activation and monitoring the magnitude of the potential
difference dV, the depositing state of the waterdrop can be discriminated.
If the predetermined voltage or more is applied even in a state
where the output V3 of the differential amplifier 15 is limited
to VB2 the waterdrop can be evaporated after the elapse of the
predetermined time T1. Even if there is the waterdrop, the deterioration
or breakage of the exothermic resistor can be prevented and, at
the same time, by eliminating the waterdrop, the operation can be
certainly returned to the stable constant temperature control operation
after the elapse of the time T2. The duration of the time T2 varies
depending on the depositing state or amount of waterdrop. It is
sufficient to set a predetermined electric power level according
to the area of the exothermic resistor to a voltage (or current)
at which it can be limited to a value of a heat flux of, for example,
4.times.10.sup.5 W/m.sup.2 or less so as not to cause the state
called "burn-out"mentioned above.
The operation in the case where the initial temperature of the
deposited waterdrop varies depending on the ambient temperature
will now be described. FIG. 5 shows a state where in the case where
the front surface of the exothermic resistor is covered by a waterdrop,
the higher the temperature of the waterdrop (regarded in a manner
similar to the suction temperature) is, the smaller the value of
the heat flux causing the burn-out is. In other words, it means
that when the temperature is high, even if the electric power which
is applied to the exothermic resistor is small, the burn-out more
easily occurs. Therefore, by using the resistor having the temperature
characteristic as a resistor 62 in FIG. 1 as shown in FIGS. 6A and
6B, the limiting voltage can be also varied in accordance with the
suction temperature. In this case, when the suction temperature
in FIG. 6A is low, a limiting voltage VB3 is increased (a difference
between the limiting voltage and the voltage at the time of no limitation
is small). In this case, when the suction temperature in FIG. 6B
is high, a limiting voltage VB4 is decreased (the difference between
the limiting voltage and the voltage at the time of no limitation
is large). By this method, the deterioration or breakage of the
resistor due to the burn-out or the like in various modes can be
prevented.
Even if the fuel flows at the maximum flow rate and the heating
temperature of the exothermic resistor decreases, the potential
difference dV instantaneously increases and the bridge applied voltage
is limited. However, since the heating temperature is soon recovered
in the ordinary air, the potential difference dV decreases and the
limitation of the bridge applied voltage is soon eliminated. Therefore,
there is no practical problem.
According to the embodiment, particularly, by detecting the depositing
state of the waterdrop and limiting the applied voltage, there are
effects such that the deterioration of the reliability such as deterioration
or breakage of the exothermic resistor can be prevented and the
measuring range of the flow rate can be widened. Although the circuit
is not particularly shown in the diagram, even in case of using
a circuit construction such that a plurality of bridge circuits
of the exothermic resistor are provided and a reverse current can
be detected, it is sufficient to add a voltage limiting circuit
of the invention every bridge circuit. Similarly, even in case of
using an indirect-heated type construction in which the exothermic
resistor and the bridge circuit of a constant temperature control
circuit are separately provided, similar effects can be obtained
by detecting the potential difference of the bridge and adding the
voltage limiting circuit of the invention to the exothermic resistor.
Although the limitation of the voltage has been described in the
embodiment, similar effects can be also obtained by limiting the
current which is supplied to the bridge.
FIG. 7 shows an example of a pattern in the case where the exothermic
resistor 11 which is used is constructed by forming a thin film
onto the silicon semiconductor substrate. An exothermic resistor
211a is vertically elongated, has a pattern in which a resistor
is turned back, and has a structure in which resistors 211d, 211e,
211f, and 211g for detecting a temperature are arranged on both
sides of the pattern. Each of the exothermic resistor 211a and the
temperature detecting resistors 211d, 211e, 211f, and 211g is formed,
for example, by arranging a resistor onto a diaphragm structure
portion having a small heat capacity etched from the back side of
a silicon substrate. A temperature compensation resistor 211c is
arranged at a location where it is hardly influenced by the temperature
by the heating of the exothermic resistor 211a. Dimensions of the
diaphragm in case of reducing the heat capacity are set such that,
for example, a length in the vertical direction lies within a range
from 1.8 .mu.m to 3 .mu.m, a width lies within a range from 0.5
.mu.m to 0.8 .mu.m, and a thickness lies within a range from 1 .mu.m
to 3 .mu.m. By setting the dimensions as mentioned above, the heat
capacity is small and a strength of the diaphragm can be easily
assured. FIG. 8 shows a cross sectional structure taken along the
line VIII--VIII of the exothermic resistor in FIG. 7. The diaphragm
has a structure such that a thickness of the place where the resistor
pattern exists is largest. It is considered that a size of waterdrop
having a possibility that it is deposited onto the exothermic resistor
through an air filter of an automobile is usually smaller than a
size (for example, 20 .mu.m) of a mesh of the air filter. However,
in the case where a dew condensation due to a sudden temperature
change or the waterdrop formed via the filter is collected by some
form in accordance with a shape or the like of the filter and grows,
it is considered that the waterdrop of a size of up to about several
ml is deposited. For example, in the diaphragm according to the
foregoing example, when a waterdrop of 1 ml is deposited, it is
considered that it covers an area in a range from about 1/5 to 1/2
of the exothermic resistor of the diaphragm. In such a case, an
air bubble is more easily produced by the burn-out.
FIG. 9 shows the second embodiment in case of using such a resistor
pattern. The hot wire driving circuit 1 is constructed in such a
manner that a current flowing in the exothermic resistor 211a is
adjusted by the differential amplifier 15 and transistor 16 so that
the potential difference at the middle point of the bridge is equal
to zero by a hoisten-bridge circuit constructed by the exothermic
resistor 211a, temperature compensation resistor 211c, and resistors
13 and 14.
The resistors 211d, 211e, 211f, and 211g for detecting a temperature
construct a bridge circuit by applying a voltage Vcc to a power
source circuit 5. As a voltage which is applied to the bridge circuit,
it is also possible to form a ratio metric construction using an
external voltage. This differential voltage is amplified by a differential
amplifier 2131 and resistors 2132 2133 2134 and 2137 around a
virtual zero point voltage V of, as a center, obtained by dividing
a reference voltage Vref by resistors 2135 and 2136. Although the
temperature detecting resistors 211d, 211e, 211f, and 211g detect
the heating temperature of the exothermic resistor 211a, since a
temperature distribution becomes uniform in the case where there
is no air flow, an output Vbi of the differential amplifier 2131
is equivalent to the virtual zero point voltage V of. When an air
flow occurs here, by arranging the exothermic resistor 211a so that
its longitudinal direction is perpendicular to the direction of
the air flow, a temperature difference occurs between the resistors
211d and 211e and the resistors 211f and 211g, so that a flow rate
signal according to the direction of the flow can be obtained as
an output Vbi of the differential amplifier 2131.
In the embodiment, particularly, the output Vbi of the differential
amplifier 2131 is inputted to an analog/digital converter 221 of
a microcomputer 220 a variation or the like of an output sensitivity
is adjusted as a digital amount by an arithmetic operation, and
a resultant signal is sent to an engine control unit or the like
as an output voltage Vout of a digital/analog converter 224. The
microcomputer 220 is constructed by: an arithmetic operating circuit
222 comprising a CPU 222a and memories, for example, an RAM 222b
and an ROM 222c; an oscillator (OSC) 226; an input/output (I/O)
unit 225; a memory, for example, a PROM 223; and the like. It is
sufficient to use a memory which can record a variation or the like
of an output sensitivity of each sensor as an adjustment value one
or more times as a PROM 223. The PROM 223 is not limited only to
an electrically writable EEPROM, a flash ROM, or the like.
Voltages Vr1 and Vr2 at both ends of the exothermic resistor 211a,
an output Vr3 of the differential amplifier 15 and the output Vbi
of the differential amplifier 2131 are inputted to the analog/digital
converter 221 of the microcomputer 220. In the microcomputer 220
the heating state of the exothermic resistor 211a is detected by
obtaining the heating temperature by an arithmetic operation from
the voltages Vr1 and Vr2 at both ends of the exothermic resistor
211a, an NPN transistor 43 is turned on by the output V6 of the
I/O unit 225 and the heating state of the exothermic resistor 211a
is limited by the voltage limitation switching circuit 6. Particularly,
an SCI (Serial Communication Interface) 227 for communicating with
the engine control unit or the like is provided for the microcomputer
220.
The voltage limitation switching circuit 6 is constructed by: the
resistors 61 and 63 serially connected between the battery voltage
VB of the power source 101 and the ground; the Zener diode 67 connected
to a node of a resistor 211h (62) and the resistor 63; the diodes
65 and 66 connected to a node of the resistors 61 and 62; a resistor
64 connected to the battery voltage VB; the NPN transistor 43; and
the like. The resistor 211h can be constructed in a manner similar
to the temperature compensation resistor 211c.
The voltage limitation switching circuit 6 limits the output V3
of the differential amplifier 15 to a voltage which is set by the
Zener diode 67 via the diode 66 and resistor 62. Since the NPN transistor
43 is OFF in the normal operation, the diode 65 is connected to
the battery of the power source 101 via the resistor 64. Thus, in
the normal operation, the output V3 of the differential amplifier
15 is not subjected to the voltage limitation. On the other hand,
by detecting the heating temperature of the exothermic resistor
and discriminating the heating state, the NPN transistor 43 is turned
on and the output V3 of the differential amplifier 15 can be limited
to the voltage set by the Zener diode 67 via the diode 66 and resistor
62.
The details of the second embodiment will now be described with
reference to FIGS. 10A to 10F. It corresponds to a case where the
exothermic resistor 211a is heated from the ordinary temperature
and the constant temperature control of about 200.degree. C. is
made as in case of turning on the power source. The case of the
normal operation in FIGS. 10A to 10C will be described. When the
maximum current which is limited by the battery voltage VB flows
in the exothermic resistor 211a, the exothermic resistor 211a is
rapidly heated (FIG. 10A). A resistance value Rh1 increases due
to the temperature elevation, the bridge circuit is balanced, and
the output V3 of the differential amplifier 15 decreases (FIG. 10C).
The time T1 during which the maximum current flows in the exothermic
resistor 211a is determined only by the resistance value Rh1 and
heat capacity of the exothermic resistor 211a and the battery voltage
VB in the ordinary case. The smaller the heat capacity is, the shorter
the time T1 is. The microcomputer 220 monitors an exothermic temperature
Th of the exothermic resistor 211a, thereby detecting a temperature
change dth1 (FIG. 10A) per predetermined time. When the temperature
change dth1 per predetermined time is larger than a predetermined
value or when the exothermic temperature Th of the exothermic resistor
211a enters a target heating temperature range, the operation is
regarded as a normal operation and the output V6 of the I/O unit
225 is held at the low level.
The case of the voltage limiting operation in FIGS. 10D to 10F
will now be described. In the exothermic resistor 211a of a small
heat capacity, particularly, if a substance which absorbs the heat
generation of a fouling, water, or the like is deposited onto the
exothermic resistor portion, an apparent heat capacity increases
and the exothermic temperature Th slowly increases until the fouling,
water, or the like is volatilized (FIG. 10D). The maximum current
continuously flows in the exothermic resistor 211a. In this instance,
a portion which locally becomes a high temperature in the exothermic
resistor 211a appears depending on the depositing state of the fouling,
water, or the like, and there is a possibility that the resistor
formed on the thin film deteriorates or the breakage of the thin
film is caused by a thermal shock. Therefore, the microcomputer
220 monitors the exothermic temperature Th of the exothermic resistor
211a, thereby detecting a temperature change dth2 (FIG. 10D) per
predetermined time. When the temperature change dth2 per predetermined
time is smaller than a predetermined value and when the exothermic
temperature Th of the exothermic resistor 211a is equal to or less
than the target heating temperature range, the operation is regarded
as a voltage limiting operation and the output V6 of the I/O unit
225 is changed from the low level to the high level and held at
the high level for a predetermined time (FIG. 10E). Thus, the NPN
transistor 43 is turned on and the voltage V3 to be applied to the
bridge circuit can be reduced to VB2 (FIG. 10F). When the applied
voltage V3 to the bridge circuit decreases, the exothermic temperature
Th of the exothermic resistor 211a slowly rises. By decreasing the
voltage, the burn-out state can be prevented and the sudden increase
or decrease in temperature can be prevented. A discriminating level
of the temperature can be freely set in the microcomputer 220. When
a heating limit time T3 elapses, the waterdrop is evaporated, the
output V6 is changed from the high level to the low level, the NPN
transistor 43 is turned off, and the operation is returned to the
normal operation. Although the time T2 which is required until the
output V3 of the differential amplifier 15 is finally stabilized
becomes long by the execution of the voltage limiting operation,
the deterioration or breakage of the resistor due to the sudden
heating temperature change can be prevented, and the time T2 can
be reduced to a time shorter than that which is required when the
exothermic resistor 211a of a large heat capacity is used. Such
a voltage limitation operating state apparently becomes a dead zone
of the sensor and cannot be distinguished. However, by using the
microcomputer 220 information showing that the voltage limitation
operating state due to the deposition of the waterdrop corresponds
to the protecting operation of the sensor and is a special state
instead of a failure of the sensor is sent to the engine control
unit or the like as an upper computer as diagnosis information of
the sensor (an analog signal at the High or Low level, a digital
signal such as an SCI [Serial Communication Interface], or the like),
thereby enabling an erroneous operation or the like on the engine
control to be prevented.
As described in the above embodiment, by setting the resistor 211h
to a resistor having a similar temperature coefficient arranged
on the same substrate as that of the temperature compensation resistor
211c as shown in FIG. 11 the temperature characteristic can be
also provided for the limiting voltage.
According to the embodiment, particularly, in case of using the
exothermic resistor of a small heat capacity, the heating state
of the exothermic resistor can be accurately monitored. By finely
limiting the heating state, there are effects such that the deterioration
or breakage of the resistor can be prevented and the reliability
of the hot-wire type air flow meter can be improved for a long time.
The third embodiment of the invention will be described with reference
to FIG. 12. In the embodiment, in a manner similar to FIG. 9 the
hot wire driving circuit 1 is constructed in such a manner that
a hoisten-bridge circuit is constructed by the exothermic resistor
211a, temperature compensation resistor 211c, and resistors 13 and
14 and a current flowing in the exothermic resistor 211a is adjusted
by the differential amplifier 15 and transistor 16 so that the potential
difference at the middle point of the bridge is equal to zero by
the hoisten-bridge circuit. A resistor 211 is constructed by forming
a thin film onto the silicon semiconductor substrate.
The voltages Vr1 and Vr2 at both ends of the exothermic resistor
211a, a voltage Vr4 of the temperature compensation resistor 211c,
and the output Vbi of the differential amplifier 2131 are inputted
to the analog/digital converter 221 of the microcomputer 220. In
the microcomputer 220 the heating temperature of the exothermic
resistor 211a is obtained by an arithmetic operation from the voltages
Vr1 and Vr2 at both ends of the exothermic resistor 211a, and a
suction temperature is obtained by an arithmetic operation from
the voltages Vr4 and Vr1 of the temperature compensation resistor
211c, thereby detecting the ambient temperature (suction temperature)
and the heating state of the exothermic resistor 211a. Further,
the heating state of the exothermic resistor 211a is limited by
the voltage limitation switching circuit 6 in accordance with an
output VB of a digital/analog converter 224b. In the voltage limitation
switching circuit 6 a differential amplifier 6101 for amplifying
the output V8 of the digital/analog converter 224b is connected
to the ground by network resistors 6102 6103 6104 and 6105 and
resistors 6106 and 6107 for dividing the output voltage. In a manner
similar to the foregoing embodiment, in the voltage limitation switching
circuit. 6 the output V3 of the differential amplifier 15 is limited
to an output of the differential amplifier 6101 which is set by
the output V8 of the digital/analog converter 224b via the diode
66 and resistor 6106.
In the normal operation, the output of the differential amplifier
6101 is generated as a maximum value. Thus, the output V3 of the
differential amplifier 15 is not subjected to the voltage limitation
in the normal operation. On the other hand, by detecting the heating
temperature and suction temperature of the exothermic resistor 211a
and discriminating the heating state, the output V8 of the digital/analog
converter 224b is set in accordance with the suction temperature.
The output V3 of the differential amplifier 15 can be digitally
and freely limited to the voltage which has already been set via
the diode 66 and resistor 6106.
Similarly, in case of using the structure such that the temperature
detecting resistors 211d, 211e, 211f, and 211g are arranged on both
sides of the exothermic resistor 211a, the local heating temperature
of the exothermic resistor 211a can be also measured by intermediate
voltages Vb1 and Vb2 between the temperature detecting resistors
211e and 211f and between the temperature detecting resistors 211g
and 211d. Usually, when a waterdrop is deposited onto the exothermic
resistor 211a heated to 100.degree. C. or higher, the temperatures
of the temperature detecting resistors 211d, 211e, 211f, and 211g
are changed in accordance with the depositing state, respectively.
For example, the fundamental depositing state of the waterdrop is
discriminated by the heating temperature of the exothermic resistor
211a. In this instance, the size or the like of the waterdrop can
be discriminated by observing the temperature changes of the temperature
detecting resistors 211d, 211e, 211f, and 211g. The time which is
required until there is no waterdrop is presumed and the time corresponding
to the dead zone of the sensor is sent to the engine control unit.
On the basis of it, the state of the sensor is discriminated, the
output of the sensor is temporarily lightened and used for control
or the like. Thus, a use efficiency of the sensor is improved and
the engine control can be more finely made.
According to the embodiment, particularly, in case of using the
exothermic resistor of the small heat capacity, the heating state
of the exothermic resistor can be accurately monitored. By finely
limiting the heating state, there are effects such that the deterioration
or breakage of the resistor can be prevented, the reliability of
the hot-wire type air flow meter can be improved more for a long
time, and matching performance with the engine system is improved.
Subsequently, FIG. 13 shows an example in the case where the resistor
211 constructed by forming a thin film onto the silicon semiconductor
substrate is mounted on, for example, a multilayer ceramic substrate
or the like. The resistor 211 on the silicon substrate is mounted
in a shaved frame of a ceramic substrate 300 and connected to a
patterned circuit 302 on the ceramic substrate 300 via an electrode
pad. Besides, an IC 303 obtained by integrating a circuit and a
heater (second resistive exothermic element) 301 for heating of
the ceramic substrate 300 are provided on the silicon substrate
300. The heater 301 for heating is arranged so as to surround the
resistor 211 on the silicon substrate and controlled in accordance
with the heating state of the first exothermic resistor 211a on
the ceramic substrate. Although the second resistive exothermic
element 301 can be provided around the first exothermic resistor
211a on the silicon substrate, it is desirable to provide it outside
of the diaphragm so that the whole resistor 211 on the ceramic substrate
can be heated. By heating the peripheral portion of the first exothermic
resistor 211a in accordance with circumstances as mentioned above,
when a waterdrop is deposited, the time which is required until
the waterdrop in the portion other than the diaphragm is eliminated
can be shortened. The ambient temperature is detected and, in case
of a low temperature, the whole resistor 211 on the silicon substrate
is first preheated by the second resistive exothermic element 301.
Thus, for example, the heating time of the first exothermic resistor
211a at the time when an ice is deposited can be remarkably reduced.
Particularly, it is effective in the case where the electric power
which is supplied to the first exothermic resistor 211a is limited
so as not to break the diaphragm.
According to the embodiment, by controlling the heating temperature
of the second resistive exothermic element in accordance with the
ambient temperature, the time of the dead zone of the hot-wire type
air flow meter due to the limitation of the heating state can be
reduced without breaking the first exothermic resistor 211a on the
diaphragm.
There is an effect such that the performance can be improved while
assuring the reliability as mentioned above.
The third embodiment of the invention will now be described with
reference to FIG. 14. A hoisten-bridge circuit based on the constant
voltage Vcc driving is constructed by: the exothermic resistor 11;
a heating portion of the current detecting resistor 13; a temperature
detecting resistor (heating temperature sensor) 111 arranged on
the side of the exothermic resistor 11 so as to be mounted on the
same substrate as that of the exothermic resistor 11; the temperature
compensation resistor 12; and the resistors 13 and 14. The hot wire
driving circuit 1 is constructed in such a manner that a current
flowing in the exothermic resistor 11 is adjusted by the differential
amplifier 15 and transistor 16 so that the potential difference
at the middle point of the bridge is equal to zero by the hoisten-bridge
circuit. When the heating temperature of the exothermic resistor
11 is low, the output of the differential amplifier 15 increases
and the apparatus operates so as to heat more. With this construction,
the resistance value of the exothermic resistor 11 is controlled
so as to be constant, namely, the temperature is set to a predetermined
value irrespective of the air flow rate. In this instance, the signal
corresponding to the air flow rate by the exothermic resistor 11
is inputted to the zero span circuit 2. The zero span circuit 2
is constructed by the differential amplifier 21 and resistors 22
23 24 25 26 and 27. The above construction is a mere example
and a system for detecting the flowing direction by using the temperature
difference as described above can be also used as output means of
the signal corresponding to the air flow rate.
Particularly, the differential amplifier 68 is prepared as second
bridge detecting means for detecting the potential difference at
the middle point of the bridge in order to detect the depositing
state of the waterdrop, and the simplified voltage limitation switching
circuit 6 is provided as a second limiter for limiting the output
voltage V3 of the differential amplifier 15 in accordance with a
detection result.
The simplified voltage limitation switching circuit 6 is constructed
by the diode 66 a resistor 69 and the like. The output of the
differential amplifier 15 is connected to the output of the differential
amplifier 68 through the resistor 17 diode 66 and resistor 69.
The voltage to be limited is determined by a voltage dividing ratio
of the resistors 17 and 69.
Even in such a construction in which the heating unit and the bridge
circuit of the constant temperature control are separately provided,
effects similar to those in the above embodiment can be obtained
by detecting the depositing state of the waterdrop and limiting
the voltage in accordance with the output state of the differential
amplifier 68 in a manner similar to the above embodiment.
According to the embodiment, since the applied voltage to the exothermic
resistor can be limited by the simple circuit construction, there
is such an effect that the reduction of the reliability such as
deterioration or breakage of the resistor can be prevented by low
costs.
Even in case of controlling the applied voltage V2 of the bridge
by the pulse driving, an effect similar to the effect which is obtained
by the linear voltage control described in the embodiment so far
can be obtained. For example, by limiting either the applied voltage
V2 to the bridge or the driving current to the predetermined value
and by limiting the pulse width corresponding to the magnitude of
the limited voltage (or current) to a predetermined value, the deterioration
or the like of the resistor due to the overheating can be prevented.
Although not shown, as another embodiment, there is an internal
combustion engine constructed by: a suction pipe in which the air
including liquid droplets flows; an air flow meter having a protecting
circuit in the case where the liquid is deposited to a detecting
device; a fuel injection valve; and a controller for controlling
the fuel injection valve on the basis of an output of the air flow
meter.
According to the embodiment, particularly, in case of using the
exothermic resistor of the small heat capacity, by preventing the
maximum heating state from continuing in excess of the inherent
heat capacity of the exothermic resistor, the deterioration or breakage
of the resistor can be prevented and there is such an effect that
the reliability of the hot-wire type air flow meter can be assured
and, at the same time, the response speed upon activation of the
air flow rate when the power source is turned on can be improved.
Thus, there are such effects that the air flow rate at the start
of the engine of the automobile can be rapidly measured and a quantity
of exhaust gas at the time of start of the engine can be reduced.
According to the embodiment, particularly, in case of using the
exothermic resistor of the small heat capacity, by distinguishing
and detecting the case where a waterdrop, an oil droplet, or the
like is deposited onto the exothermic resistor portion and the normal
case where there is no deposition, there are such effects that the
heating state is limited only in the case where the waterdrop, oil
droplet, or the like is deposited on the exothermic resistor portion
without narrowing the inherent measuring range of the flow rate,
the deterioration of the resistor can be prevented, and the reliability
of the hot-wire type air flow meter can be assured and, at the same
time, the response speed in response to the leading edge of the
air flow rate when the power source is turned on can be improved.
There are, consequently, such effects that the air flow rate at
the start of the engine of the automobile can be rapidly measured,
and a quantity of exhaust gas at the start of the engine can be
reduced. |